1. Field of the Invention
This invention relates generally to the field of geophysical prospecting and more particularly to a method and apparatus for sensing when a seismic gas exploder is in contact with the earth.
2. Description of the Prior Art
The invention to be described is of primary interest in connection with the control and operation of so-called seismic gas exploders. Typically, these are devices resting on the ground wherein a gas explosion takes place within an expansible chamber formed between a bottom impact mass and a movable piston assembly causing the impact mass to move downwardly to generate a seismic wave within the earth. The piston assembly moves in an opposite direction, followed by the upward rebound of the entire exploder housing. When operating such exploders, the possibility exists that the exploder may be fired when it is not in contact with the earth. Some versions of gas exploders cannot withstand such a stress safely. Furthermore, the resultant seismic pulse may differ substantially from one generated when the exploder interfaces the earth. Thus, there is a need for indicating reliably when the exploder has returned to earth so that such firing does not occur prematurely, for example as a result of malfunction or by attempting to employ a firing sequence more rapid than the system can tolerate.
As a corollary to the above, the Applicant has appreciated that it would be advantageous if a gas exploder could be made self-actuating so that its contact with the earth after firing would automatically trigger the next firing sequence. This would eliminate the need for a separate timer for controlling the firing circuit.